The association between the mid–latitude circulation and rainfall over the Indian region on an intraseasonal time–scale is investigated by considering 11 years (1974–1984) of Northern Hemisphere 500 hPa geopotential heights and rainfall data for the Indian summer monsoon months June through to September. On the basis of extensive correlation analysis between the geopotential heights and rainfall, it is seen that three regions over the mid–latitudes, the Manchurian region, the Algerian region and the Caspian sea region show positive correlation with rainfall over India, with higher values north of 20°N latitude. Lead and lag correlations between the heights at the locations identified above and rainfall over India reveals that some common element of low–frequency variability is influencing the mid–latitude circulation and Indian rainfall. On the interannual scale the connections between the winter–time low–frequency patterns (the Pacific/North Atlantic, the West Pacific Oscillation, the North Atlantic Oscillation and the Eurasian) and Indian summer monsoon rainfall (ISMR) are investigated. Only the West Pacific Oscillation pattern shows a significant relationship with the ISMR. Further, the interannual and the decadal variability is examined by using the Northern Hemisphere zonal index data for the period 1900–1993. Results reveal that the decadal–scale variability of the ISMR and the circulation features of the Northern Hemisphere are connected. © 1997 The Royal Meteorological Society.

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Seasonal (June through September) percentage departure from normal rainfall patterns over India have been classified by objective methods utilizing data for a 120-year period . The methods used are the map-to-map correlation method and the k-means clustering method. Empirical orthogonal functions (EOFs) are also presented.It is seen that 75 per cent of the charts can be cIassified into six distinct types by the map-to-map method. Six distinct types also are obtained by the k-means method. The first four dominant EOFs explain 52 per cent of the variance.Intercorrelations and contingency tables among the types obtained by these methods and visual examination suggests that the dominant patterns are similar, irrespective of the method used. It is also seen that the monsoon rainfall for the country as a whole and the spatial patterns may have no relationship. Even during normal monsoon years, the spatial patterns may be different.The advantages and disadvantages of the methods used are discussed.

In this study, 20 years (1961)(1962)(1963)(1964)(1965)(1966)(1967)(1968)(1969)(1970)(1971)(1972)(1973)(1974)(1975)(1976)(1977)(1978)(1979)(1980) of rainfall data for 34 stations over Thailand are used to investigate and understand the intraseasonal and interannual variability of the summer monsoon. Dominant structures of 5-day and seasonal rainfall are determined through empirical orthogonal functions (EOFs). Monthly and seasonal spatial patterns and a map showing the coefficient of variation are also presented.On an intraseasonal time-scale regions with high 30-60-day variances (Madden Julian oscillations-MJOs) and 1&20-day variances (Quasi-biweekly oscillations-QBWOs) are identified using a band-pass Butterworth filter for 5-day rainfall. It is seen that the MJOs are dominant over the Indian region and the QBWOs are dominant over the Thailand region. Extended EOFs (EEOFs) have shown that the most important evolutionary feature over India is the northward propagation associated with the MJOs, whereas over the Thailand region it is westward, associated with the QBWOs. Northward propagation of rainfall anomalies is not observed over the Thailand region.On the interannual scale the area over north-west Thailand is related well with rainfall variation over west-central India. The Darwin pressure tendency (DPT) and the subtropical ridge over India shows significant relation with rainfall over northwest Thailand. However, the Northern Hemisphere surface temperature (NHST), Quasi-biennial Oscillation (QBO), and West Pacific ridge (WPR) show no significant relation.